From Open German Specification (Offenlegungsschrift) DT-OS No. 2,304,496 and the corresponding U.S. Patent No. 3,914,387 it is known to remove SO.sub.2 from exhaust gases by scrubbing with an absorbent solution which contains caustic soda (NaOH) and sodium sulfate. In that process, part of the used absorbent solution is continuously regenerated by the addition of calcium hydroxide with precipitation of calcium sulfate and is re-used.
While this technique has been found to be highly effective, it nevertheless has the drawback that, as described, it cannot also rid the exhaust gas, usually derived from combustion in an industrial furnace or power plant or from a chemical or metallurgical process plant, of fluorine compounds such as HF before the gas is recycled or discharged into the environment. Thus, if the exhaust gas contains substantial quantities of HF, special means must be adopted for its removal and such means are seldom economical.